1. Field of the Invention
The invention relates to polyvinyl-alcohol-stabilized redispersible powders having plasticizing properties, to processes for preparing them, and to their use in chemical products for the construction industry.
2. Background Art
Polymer powders stabilized with protective colloid which are redispersible in water (redispersible powders) are employed as additives in construction compositions, based for example on cement or gypsum, in order to improve strength and abrasion resistance, or in the case of construction adhesives, to improve adhesive tensile strength. The protective colloid used is frequently polyvinyl alcohol, since this colloid contributes to improved strength, in comparison to polymers stabilized with low molecular weight emulsifiers.
Redispersible powders of this kind are obtainable by drying the corresponding protective-colloid-stabilized polymer dispersions, with the addition during drying of so-called drying assistants (spraying aids), in order to prevent the formation of agglomerates. In EP-A 467103 water-soluble copolymers with carboxyl-functional comonomer units are used as drying assistants. DE-A 19707746 describes the use of copolymers of ethylenically unsaturated carboxylic acids and their hydroxyalkyl esters as drying assistants. EP-A 629650 discloses copolymers containing acrylamidomethylpropanesulfonic acid units as a spraying aid. EP-A 671435 recommends spray drying in the presence of copolymers containing hydrophilic, water-soluble, and salt-forming monomers.
In many cases modification of construction compositions such as mortar or concrete with redispersible powders decreases fluidity. In the case of fluid mortars such as self-leveling fillers or screeds, however, a liquid consistency which exhibits little change during processing, is essential. Consequently, in these applications, additives known as cement plasticizers are employed. For example, EP-A 549280 describes plasticizers based on graft copolymers containing four different comonomer units: (meth)acrylic acid units, hydroxyalkyl (meth)acrylate units, (meth)acrylamido-2-methylpropanesulfonic acid, and (meth)acrylates containing EO groups. JP-A 59/162161 describes water-soluble copolymers containing hydroxyalkyl esters of an ethylenically unsaturated monocarboxylic acid and ethylenically unsaturated monocarboxylic or dicarboxylic acid units. EP-A 407889 discloses redispersible powders which employ phenolsulfonic acid-formaldehyde condensation products in order to improve their fluidity when used in hydraulic binders. EP-A 812872 describes processes for preparing polymer powders with the addition of drying assistants having a plasticizing effect.